A wide variety of different sensor types are used to detect objects in places and mediums other than air. Of particular interest are sensors that are used for scanning for objects (such as mines) that have been deposited under water. Such sensors are also used to detect under water topological formations that are indicative of mineral deposits or other features of interest. One type of sensor that is used in such applications is the laser line scan (LLS) sensor.
Laser line scan sensors generate images one line at a time. This is accomplished by using four mirrors attached to a spindle that reflects returned photons into four channels, with each group of four channels having a gain applied to it. Sometimes the effective gains of the scan lines become unequal causing contrast differences between the lines. This problem is attributed to not only each mirror (representing a channel) having a slightly different reflectance property, but also because each group of four channels (representing four lines per spindle rotation) may have different gains applied to them.
One technique used to correct this line contrast difference effect is to average the mean signal values of every four lines (corresponding to a single spindle rotation). However, this technique is deficient in that the trouble frequencies that cause the line contrast difference effect are not restricted to being periodic at a rate of every fourth line. Thus, this mean line averaging technique can correct only a few of the trouble frequencies associated with line contrast difference effect.
Fourier analysis techniques have been used in the past to remove unwanted noise frequencies. However, such techniques have not been applied to LLS data, and the algorithms used were manual, and so did not automatically identify trouble frequencies. In conventional systems identification of the trouble frequencies causing the line contrast difference effect is difficult. Further, such conventional systems operated to "zero out" trouble frequencies instead of suppressing them. Consequently, conventional systems can not detect and suppress a wide range of trouble frequencies associated with line contrast difference effect. Also, conventional systems do not eliminate Gibbs Effect, which causes the slight ringing that is common with filters using a sharp cut-off. Because of these drawbacks, the conventional art can provide only limited effectiveness for laser line scanners detecting for objects in turbulent or deep water.